Typical radiation detectors have a light receiving element optically coupled with a scintillation crystal (also referred to as a crystal element). To provide positron imaging devices and PET devices of higher spatial resolution, there has been developed a DOI (Depth of Interaction) radiation detector (hereinafter, also referred to simply as a DOI detector) which can even detect the position of incidence to the crystal element in the depth direction. As shown in FIG. 1, a light receiving element 10 such as a position sensitive photomultiplier tube (PS-PMT) is provided thereon and optically coupled with a crystal block 20 which includes a three-dimensional array of a large number of crystal elements extending upward above the light receiving surface. By identifying crystal element that detect radiation, the detection position in the crystal block 20 is obtained in a three-dimensional manner.
The DOI detector is useful in identifying the three-dimensional direction in which the radiation source is. The use of the DOI detector as a radiation detector for a PET device can improve the sensitivity of the PET device without degrading the resolution.
The crystal elements in the DOI detector can be identified by various techniques. For example, crystal elements that are two-dimensionally arranged in parallel on the light receiving surface of the light receiving element 10 are identified by Anger calculations of the outputs of the light receiving element. As illustrated in FIG. 2, the response positions of the respective crystal elements appear on a two-dimensional (2D) position histogram which shows the results of the Anger calculations. The Anger calculation is a method to calculate the center of gravity or the response position of light receiving element signals, and is widely known as a method for discriminating a large number of crystal elements with a small number of light receiving elements.
To make crystal identification in the depth direction, i.e., to identify a plurality (in FIG. 1, three) of stacked layers of two-dimensional arrays 21, 22, and 23 of crystal elements illustrated in FIG. 1, there have been proposed the following techniques:
(1) As shown in FIGS. 1(a) and 1(b), scintillation crystals having different waveforms are used for the respective layers (in FIG. 1(a), LSO, GSO, and BGO; in FIG. 1(b), GSO with 1.5 mol % of Ce, 0.5 mol % of Ce, and 0.2 mol % of Ce, respectively). The layers are identified by waveform discrimination (see Patent Document 1 and Non-Patent Documents 1 and 2).
(2) A two-dimensional array of scintillation crystals typically includes a reflector between the individual crystal elements, in which case the responses of the respective crystal elements appear on the 2D position histogram at the positions where the layout of the crystal elements is reflected. Using this, as shown in FIG. 3(a), the first layer 21 is formed, for example, as a 6×6 crystal array and the second layer 22 a 7×7 crystal array so that the layers overlap each other with a displacement therebetween. Alternatively, grooves are cut from above and below the crystal block 20 to form slits 30 in the crystal arrays 21 and 22 so that the upper and lower crystal elements are laid out differently as shown in FIG. 3(b). The response positions of the respective crystal elements in the three-dimensional array are thereby separated so as to be identifiable as illustrated in the 2D position histogram of FIG. 2 (see Non-Patent Documents 3 and 4).
(3) As illustrated in FIG. 4, the reflector 32 in the two-dimensional crystal arrays 21 to 24 may be removed in part so as to control the spreading of the scintillation light. This makes it possible to control the response positions of the respective crystal elements 30. In the diagram, 34 represents a portion where there is an extremely thin layer of air without the reflector 31. Consequently, as shown in FIG. 5, the response positions of all the crystals in the three-dimensional array can be separated for identification (see Patent Documents 2 to 5 and Non-Patent Document 5).
(4) Filters for cutting off certain wavelengths are interposed between layers, and the layers are identified by the resulting wavelengths (see Patent Document 6 and Non-Patent Document 6).
(5) In some approaches, the foregoing techniques (2) and (3) are combined with the waveform discrimination (1) for multistage identification (see Non-Patent Documents 7 and 8).
Such DOI detectors are all configured to include a rectangular prism crystal or to have each element with a rectangular prism shape.
Meanwhile, there have been proposed technologies for use in two-dimensional crystal array radiation detectors that do not perform DOI detection, wherein triangular prism scintillator crystals are used as in the present invention. In any of the technologies, the crystal shape has been contrived for the purpose of closely arranging the crystal elements. The technology described in Patent Document 7 is to shape the entire detector including its crystal element and light receiving element as a triangular prism so that a large number of detectors can be closely arranged in a spherical configuration.
The technology described in Non-Patent Document 9 is to arrange several different types of crystal elements on a light receiving element of cylindrical shape with the acute angles of the triangles toward the center. Detecting crystals are identified from the waveforms, whereby the direction of the radiation source is identified.
The technology described in Patent Document 8 is to arrange detectors of rectangular prism shape into a hexagonal PET detector ring, in which case triangular prism scintillation crystals and light receiving elements are used as auxiliary infilling detectors.    Patent Document 1: Japanese Patent Application Laid-Open No. Hei 6-337289    Patent Document 2: Japanese Patent Application Laid-Open No. Hei 11-142523    Patent Document 3: Japanese Patent Application Laid-Open No. 2004-132930    Patent Document 4: Japanese Patent Application Laid-Open No. 2004-279057    Patent Document 5: Japanese Patent Application Laid-Open No. 2007-93376    Patent Document 6: Japanese Patent Application Laid-Open No. 2005-43062    Patent Document 7: Japanese Patent Application Laid-Open No. Hei 8-5746    Patent Document 8: Japanese Patent Application Laid-Open No. Hei 5-126957    Non-Patent Document 1: J. Seidel, J. J. Vaquero, S. Siegel, W. R. Gandler, and M. V. Green, “Depth identification accuracy of a three layer phoswich PET detector module,” IEEE Trans. on Nucl. Sci., vol. 46, No. 3, pp. 485-490, June 1999    Non-Patent Document 2: S. Yamamoto and H. Ishibashi, “AGSO depth of interaction detector for PET,” IEEE Trans. on Nucl. Sci., vol. 45, No. 3, pp. 1078-1082, June 1998    Non-Patent Document 3: H. Liu, T. Omura, M. Watanabe, and T. Yamashita, “Development of a depth of interaction detector for y-rays,” Nucl. Inst. Meth., A459, pp. 182-190, 2001.    Non-Patent Document 4: N. Zhang, C. J. Thompson, D. Togane, F. Cayouette, K. Q. Nguyen, M. L. Camborde, “Anode position and last dynode timing circuits for dual-layer BGO scintillator with PS-PMT based modular PET detectors,” IEEE Trans. Nucl. Sci., vol. 49, No. 5, pp. 2203-2207, October 2002.    Non-Patent Document 5: T. Tsuda, H. Murayama, K. Kitamura, T. Yamaya, E. Yoshida, T. Omura, H. Kawai, N. Inadama, and N. Orita, “A four-layer depth of interaction detector block for small animal PET,” IEEE Trans. Nucl. Sci., vol. 51, pp. 2537-2542, October 2004.    Non-Patent Document 6: T. Hasegawa, M. Ishikawa, K. Maruyama, N. Inadama, E. Yoshida, and H. Murayama, “Depth-of-interaction recognition using optical filters for nuclear medicine imaging,” IEEE Trans. Nucl. Sci., vol. 52, pp. 4-7, February 2005.    Non-Patent Document 7: S. J. Hong, S. I. Kwon, M. Ito, G. S. Lee, K.-S. Sim, K. S. Park, J. T. Rhee, and J. S. Lee, “Concept verification of three-layer DOI detectors for small animal PET,” IEEE Trans. Nucl., Sci., vol. 55, pp. 912-917, June 2008.    Non-Patent Document 8: N. Inadama, H. Murayama, M. Hamamoto, T. Tsuda, Y. Ono, T. Yamaya, E. Yoshida, K. Shibuya, and F. Nishikido, “8-layer DOI encoding of 3-dimensional crystal array,” IEEE Trans. Nucl. Sci., vol. 53, pp. 2523-2528, October 2006.    Non-Patent Document 9: Y. Shirakawa, “Whole-Directional Gamma Ray Detector Using a Hybrid Scintillator,” Radioisotopes, vol. 53, pp. 445-450, 2004.
The waveform discrimination-based method (1) is made possible by a combination of certain crystals. Problems have been pointed out, however, that it entails a discrimination error and that it lowers the time resolution and degrades the count rate characteristic of the detector. The method (2) of mutually displacing the layers needs fine adjustments of the displacement. In addition, the crystal arrays of different sizes in the upper and lower layers make it difficult to wrap the reflector around the entire crystal arrays. The method (3) using light distribution control creates wasted space on the 2D position histogram as shown in FIG. 6. The greater the distances between the crystal response positions are, the better the separation is and the more enhanced the resulting discrimination power is. The crystal response positions are therefore ideally aligned at uniform intervals on the 2D position histogram. In order to increase the distances between the crystal response positions, it is also important to reduce the spreading of each crystal response position on the 2D position histogram. For that purpose, it is needed to improve the efficiency with which the light receiving element detects scintillation light. With rectangular prism crystals, however, the efficiency is difficult to improve since the scintillation light can repeat regular reflections inside. The method (4) using a wavelength cut filter lowers the detection efficiency of the scintillation light.